1. Field of Invention
The present invention relates to a pixel circuitry of a display device, and more particularly, to a pixel circuitry that calibrates a common voltage of the display device.
2. Description of the Related Art
With great advance in the techniques of electro-optical and semiconductor devices, flat panel displays, such as liquid crystal displays (LCD), have enjoyed burgeoning development and flourished in recent year. Due to the numerous advantages of the LCD, such as low power consumption, free of radiation, and high space utilization, the LCD has become the main stream in the market. The LCD panel has no capacity of emitting light by itself so that a backlight module is disposed back to the LCD panel to provide a surface light source required by the LCD panel. The LCD panel displays the images by controlling an orientation direction of the liquid crystal to adjust a light transmittance and a light reflectance of the light source.
Generally, a pixel electrode and a common electrode are coupled to two terminals of the liquid crystal layer, respectively. A voltage of the pixel electrode varies with pixel signals, and the common electrode may be coupled to a common voltage. As known, the orientation direction of the liquid crystal is controlled by a voltage difference between two terminals of the liquid crystal layer and the electric field direction applied on the liquid crystal layer. In order to avoid liquid crystal polarization, polarity inversion is often performed on the LCD. Namely, voltages with different polarities, such as positive polarity and negative polarity, are alternatively used for driving liquid crystal at different time. Whether the voltage applied to the liquid crystal is the positive polarity or the negative polarity is determined according to the electric field direction applied on the liquid crystal. If the voltage of the pixel electrode is greater than the common voltage, the liquid crystal is driven with the positive polarity voltage. Otherwise, the liquid crystal is driven with the negative polarity voltage.
However, human eyes may perceive flickers on the display panel while performing polarity inversion since the common voltage provided to the common electrode would be affected by ambient temperature and then causes the positive polarity voltage and the negative polarity voltage are not symmetric. Generally, additional devices, such as a photo sensor, a flicker checker, and some related devices, are needed to adjust the common voltage before the LCD is shipped out from the factory. When the LCD is turned on, the photo sensor senses lights, which transmits through the liquid crystal and emits to the display panel, and thereby generates an electronic signal to the flicker checker. When the flicker checker shows that the electronic signal overly rises and falls, the flickers may occur on the display panel, so the common voltage should be manually calibrated. For example, a variable resistor or a digital to analog converter associated with the common voltage is adjusted.
The operations of sensing the lights and adjusting the common voltage should be carried out until the LCD system gets into thermal equilibrium for ensuring the calibration of the common voltage is correct. As a result, the calibration of the common voltage consumes time and hardware cost. In addition, inaccuracy calibration may occur during manual adjustment process.